Now
don't panic. Somewhere, deep in the galaxy, a huge demolition ball of
gas and dust is racing straight towards us. It's so dark you can't see it, but
it's there all the same. What's worse: when it visits our solar system, it
will create havoc and destruction on Earth.

`Nemesis',
is the ominous name scientists came up with when they began speculating about
the sinister space body in 1984. They couldn't have picked a better name: if
Nemesis really exists, it should be one of the worst enemies of life on earth.
Our solar system is what Nemesis calls `home'. And the bad part is: home is
where Nemesis is heading.

What
Nemesis is? It's nothing less than our Sun's twin brother. Yes, that's right:
a second Sun! In the cosmos, many stars come in pairs. And according to the
Nemesis theory, our `star' -- the Sun -- is no exception.

But
wait -- why then do we see only one Sun? Well, for one thing, Nemesis is
REALLY far away at the moment. Nemesis
orbits the Sun in a huge ellipse, with its furthest point a distance of three
light years away. But once every 25 to 30 million years, Nemesis closes in on
the Sun. It slams into the outer regions of our solar system: a place called
the Oort Cloud, at a distance of about half a light year away from the
Sun.

Well,
if Nemesis really is a star, we'll surely see it coming, right? Wrong. In
fact, the Sun we're talking about here doesn't shine at all. Nemesis is what
is known as a brown dwarf, a dark ball of matter that has never `lit
up' to become a Sun. But don't underestimate it: Nemesis is still as big as
several Jupiters, enough to dwarf the Earth many, many times.

Oh
yes, we all know the dinosaurs were smoked by a huge piece of space debris the
size of San Francisco slamming into our planet some 65 million years ago. But
there's more. Around 35 million years ago, another meteoric
bombardment hit our planet. Not to mention the cataclysmic, yet unexplained
events that led to the extinction of almost all life on Earth 251 million
years ago. In fact, the path of evolution is littered with mass extinctions.

In
1984 it suddenly dawned on two University of Chicago paleontologists, David
Raup and John Sepkoski, that these extinctions actually show a certain
pattern. Raup and Sepkoski had put together a detailed list of sea life
that had become extinct during the past 250 million years, containing more
than 3,500 different species. And there it was: according to the lists, our
planet faces a period of death and destruction every 26 to 30 million years.

In
the years that followed, scientists from all over the world speculated on the
cause of this bizarre extinction pattern. Did the gravitational pull of huge
planets like Jupiter and Saturn have something to do with it? Or was there a
mysterious `Planet X' somewhere out there, orbiting our solar system and
paying us a visit every 26 to 30 million years? Then, in the mother of all
science magazines Nature, two groups of researchers independently of
each other came up with the twin Sun-idea. It was 1984, and the Nemesis
hypothesis was born.

Although
a lot of astronomers at first thought the Nemesis idea was a joke, the
hypothesis also attracted the attention of a considerable number of respected
scholars. One group of researchers, led by the Louisiana-based astrophysicist
Daniel Whitmore, even claimed Nemesis might be a black
hole, a super-dense
dead sun that eats all matter and light it encounters.

Meanwhile,
Walter Alvarez -- the guy who came up with the dinosaur extinction theory --
studied a number of impact craters on earth. His conclusion: indeed, once
every 26 to 30 million years, something starts throwing an awful lot of rocks
at our planet. Other researchers -- among whom the `inventor' of Nemesis
Muller himself -- studied the age of impact craters on the moon, and reached
similar conclusions.

So,
is Nemesis for real? In spite of all the evidence, most astronomers don't
think so.

For
one thing, Nemesis has never been spotted. Okay, it's dark and relatively
small. But still, we should have noticed something, studying the
motion of the Sun. If something really orbits the Sun, it tugs a little at
the Sun, no matter how far away it is. And no one has ever noticed any
unexplained wobbling of the star we call Sun.

Besides,
the `extinction periods' themselves are heavily disputed. The fossil record
dates back about four billion years and is scattered with mass extinctions. On
the whole, there's just no real time pattern there.

Still,
in the beginning of 2000 the Nemesis theory quite unexpectedly gained some new
ground, when a team of distinguished US astronomers of Berkeley and Princeton
calculated that Nemesis, if it exists, indeed should be a brown dwarf.
Meanwhile, John Matese of the University of Louisiana studied the orbits of 82
comets in the Oort Cloud. According to Matese, their orbits had some elements
in common that could only be explained if the comets had been influenced by
the gravitational pull of an object several times the size of Jupiter.

So
don't write off Nemesis yet. The case on the Sun's evil brother
still isn't closed. Luckily, we have plenty of time to learn more about it.
The Death Star is due to arrive a royal thirteen million years from now.